1. Field of the Invention
The present application relates to a resonant circuit applicable to an oscillator.
2. Background Arts
Conventional communication systems usually implement a voltage controller oscillator (VCO), which includes a resonant unit and an amplifier, for generating an oscillating signal. The resonant unit often has a type of a micro-strip line because of relatively restricted signal loss and relatively high Q-value thereof, which are parameters showing steepness of the resonance, in order to reduce phase noises in frequency bands of a microwave and/or a millimeter wave.
A Japanese Patent laid open No. 2008-141291A has disclosed techniques relating to microwave filters and high frequency oscillators. The microwave filters and the high frequency oscillators disclosed therein have an oscillator and the band-passing filter integrated on a quartz substrate, which inevitably has an excellent Q-value. As such, a resonant unit has been integrally formed with a semiconductor amplifier but on a quartz substrate as the communication system request the higher level integration and further reduction of cost. However, such a resonant unit formed on the semiconductor material inevitably shows an inferior Q-value because of relatively smaller phase shift of micro-strip lines formed on the semiconductor material; accordingly, the phase noise of the oscillator becomes degraded. Setting the electrical length of the micro-strip line and the angular frequency of the oscillator are θ and ω0, respectively; the Q-value is given by:
  Q  =                    ω        ⁢                                  ⁢        0            2        ⁢                                                ∂            θ                                ∂            ω                                              ω        =                  ω          ⁢                                          ⁢          0                    The reflection co-efficient S11 of the lossless transmission line having the characteristic impedance of Z0 and a grounded end is given by:
      S    11    =                    j        ⁢                                  ⁢        Z        ⁢                                  ⁢        tan        ⁢                                  ⁢        θ            -              Z        0                            j        ⁢                                  ⁢        Z        ⁢                                  ⁢        tan        ⁢                                  ⁢        θ            +              Z        0            which means that, on the smith chart, the reflection co-efficient S11 in the S-parameter rotates by 20 measured from the origin without varying a diameter. The electrical length θ is given by:θ=(2π/λg)L, λg=2πc′/ω∴θ=ωL/c′where L, c′, and λg are an electrical length of the micro-strip line, propagation speed, and a wavelength of a signal transmitted on the micro-strip line, respectively.
That is, the signal frequency and the electrical length of the transmission line may determine the phase and the phase shift of the reflected signal. A key factor is that the phase shift thus configured for a micro-strip line has restricted the Q-value of the oscillator, that is, the micro-strip line integrally formed on the semiconductor material is hard to enhance the Q-value.
Various resonant units that increase the phase shift of the reflected signal have been investigated and developed. The crystal oscillator disclosed in the prior art above described has shown a high Q-value in relatively lower frequencies by utilizing the physical properties of the quartz. In a microwave band, a technique similar to those disclosed in the prior art above described may be applicable by using overtones of the oscillation. However, in further higher frequencies such as those in millimeter wavelengths, techniques similar to those above described becomes almost impossible. In such bands of the microwave band and/or the millimeter wave band, and an oscillator is also to be integrated with other devices, a resonant unit implementing primarily transmission lines such as micro-strip lines are generally used. A micro-strip line is put between capacitors with relatively small capacitance is one of such arrangements. However, those known techniques assume a transmission line with a quarter wavelength of an oscillation; the size of the resonant unit, accordingly, tends to be enlarged.